The invention relates to the field of athletic equipment. More particularly, the invention relates to a flipper device for aiding a swimmer.
Propulsion in swimming typically involves a combination of different forces. Swimmers are propelled primarily by drag forces and assisted by some lift. There is no fixed point in the water from which a swimmer may push. To move the body forward, a swimmer moves water backwards with hands and legs. Lift forces in swimming are primarily caused by the angle of attack of the hands, legs, and feet. The force thus contributed to propulsion is explained by Newton""s third law of motion, wherein for each and every action there is an equal and opposite reaction.
The movement of the leg and foot of a swimmer contributes significantly to the propulsion of a swimmer. As the surface of the foot is angled and moved during a kick, the water it encounters is deflected and forced away. Friction causes the leg and foot movements to slow, as the force of the kick is imparted to the water. A force or pressure is thus created that acts upon the surfaces of the leg and foot in an equal and opposite direction. This pressure produces the main force in swimming that propels the body forward.
Certain aquatic mammals, such as the walrus, have finned members to assist in swimming. The hind limbs of the walrus have flexible fins that are radially expandable. The fins are enlarged to provide a greater surface area on the downstroke (or power stroke) kick. The amount of water forced away from the swimming walrus is thereby maximized. The fins are contracted to provide a lesser surface area on the upstroke, minimizing the water resistance encountered. The walrus thus uses the radially expandable fins to optimize its swimming efficiency. Other marine mammals, such as seals, sea lions, and sea otters, use a variety of fore and hind fin movements to provide both propulsion and steering swimming motions.
Foot flipper or fin devices are used by swimmers to assist in their propulsion through water. A flipper is designed to catch water and push it behind to propel the swimmer. FIG. 1 is a top view of a swim flipper 10, according to the prior art. The foot is inserted into the shoe 14 of the flipper. The blade 12 extends from the shoe and provides an enlarged surface area 16 that pushes the water on both the upstroke and the downstroke kick. The enlarged surface area 16 imparts the force of a kick to a larger area of water. The resulting increased equal and opposite pressure acting upon the foot surface propels the swimmer at a faster rate through the water.
It is known to provide rigid members 18 to provide axial support to the blade. Some prior art flipper blades also have channels 17 running along the edges of the blade to facilitate water flow. However, the flippers known in the prior art have a constant width 22, and are not readily expandable to provide an increased surface area during the power kick portion of the periodic fin motion. Such prior art swim flippers maintain a constant surface area on both upstroke and downstroke kicks.
FIG. 2 and FIG. 3 are side views of a prior art swim flipper during an upstroke and a downstroke kick, respectively. An upstroke, as used herein, is a kick towards the surface of the water 24 by a swimmer performing a standard freestyle crawl stroke. As the flipper moves upward, water resistance 26 opposes the flipper""s motion, causing the blade to bend downward 28, as seen in FIG. 2. A downstroke is a kick away from the water surface. The water resistance 26 is now directed up towards the flipper, causing the blade to bend upwards 30, as seen in FIG. 3.
The features that have the most influence on the performance of a foot flipper are surface area, flexibility, and weight. Enlarged surface area imparts the force to a larger area of water, but subjects the swimmer to increased water resistance. Flexibility provides increased lift forces through the optimization of the angle of attack of the feet. Additionally, less strength and effort is required to kick with a lighter weight foot flipper.
G. Beuchat, Foot Flipper Device, U.S. Pat. No. 4,300,255 (Nov. 17, 1981), describes a shoe having a fin with a longitudinal cross-section of generally double curvature. The front portion of the shoe forms a separate assembly from the flipper. A rib that may traverse at least a portion of the concave curvature at the tip of the flipper provides axial rigidity. The first curvature is arranged to make the fin adopt a position extending along the axis of the swimmer""s leg. The second curve is adapted to make it possible, during the downstroke kick, for the tip of the flipper to conserve an angle of attack which is as close as possible to the axis of the leg, rather than the foot. [(col. 2, lines 32-35)]. During the ascending movement of the leg, the flipper is flattened against the sole of the foot to provide a propulsive force. While Beauchat recognizes that the downstroke kick provides a greater propulsive force than the upstroke, the surface area of the flipper remains constant throughout a swimming motion. Beauchat does not describe an increase of the available surface area during the downstroke kick, nor does it describe a reduction in the surface area of the flipper during the upstroke, such as to minimize the increased water resistance of the upstroke.
L. Cressi, Swimming Flipper Made of Two Different Materials, U.S. Pat. No. 4,954,111 (Sep. 4, 1990), describes a flipper having a blade whose outer sole extends under the heel of the shoe. The blade is equipped with an arched strap which surrounds the shoe transversely, providing a comfortable soft shoe material without compromising the effectiveness of the flipper. The flipper blade is formed of a harder material than the shoe. However, by extending the blade under the heel of the shoe, the deformation caused by the bending of the softer shoe material is minimized. Additionally, the outer portion of the sole is fitted with a transverse strap. The strap holds the foot against the outer sole and blade when the foot is moved upwards. While Cressi describes a fin having enhanced comfort, there is no disclosure of a structure to improve the propulsion generated by the flipper design. Furthermore, there is no disclosure of increasing the available surface area during the downstroke kick to maximize the propulsive force. As well, there is no discussion of reducing the frictional resistance during the upstroke to minimize the effort exerted by the swimmer.
P. Tomlinson, Water Surface Running Fins for the Feet, U.S. Pat. No. 4,787,871 (Nov. 29, 1988), describes a flipper device which enables a user to run on the surface of a body of water. The flipper blade is formed of a plurality of pivotally connected fingers. As the user runs, these fingers pivot to reduce water drag in preparation for the next forward motion stroke.
A. Perry and P. Mueller, Expandable Swim Flipper, U.S. Pat. No. 5,813,889 (Sep. 29, 1998), describe an expandable swim flipper which xe2x80x9cincludes a blade having at least an expandable portion thereof. The expandable portion radially expands in response to water resistance directed to a first surface of the blade. The surface area of the flipper is correspondingly increased to propel a swimmer at a faster rate through the water. The expandable portion radially contracts in response to water resistance directed to a second surface of the blade. The surface area of the flipper is correspondingly reduced, and is thereby subject to decreased water resistance.xe2x80x9d
The disclosed prior art fin systems and methodologies thus provide basic swim fins having a variety of designs. However, the prior art fin systems and methodologies fail to provide a fin blade design which offers lateral expansion and contraction, in response to applied water pressure during periodic motion of the fin. Furthermore, the prior art fin systems fail to provide a fin blade design which offers lateral expansion and contraction as well as flexion, in response to applied water pressure during periodic motion of the fin.
It would be advantageous to provide a swim flipper that mimics the hind fins of a walrus to optimize the propulsion of a swimmer through the water. The development of such a swim fin would constitute a major technological advance.
It would also be advantageous to provide a swim fin which laterally expands when moved in a first direction to provide a greater surface area, to maximize the amount of water forced away from the swimmer. The development of such a swim fin would constitute a further technological advance. As well, it would be advantageous to provide a swim fin which laterally contracts when moved in a second direction to provide a lesser surface area, to minimize the water resistance encountered. The development of such a swim fin would constitute a further major technological advance. Furthermore, it would be advantageous to provide a swim fin which provides lateral expansion and contraction, in combination with flexion, in response to applied water pressure during periodic motion of the fin. The development of such a swim fin would constitute a further major technological advance.
An enhanced swim fin is provided comprising a flipper blade having an increased effective surface area during movement in a first direction, and a decreased effective surface area during movement in a second direction. The swim fin preferably comprises a staged opening during movement in the first direction, in which the opened flipper blade provides increased flexion as the effective surface area is increased. The swim fin similarly preferably comprises a staged closing during movement in the second direction, in which the opened flipper blade provides decreased flexion as the effective surface area is decreased. In some embodiments of the staged swim fin, a central hinge, generally located longitudinally along the blade on the fin, provides staged opening and/or flexion. Various embodiments provide fin opening and closing for either forward or backward kicks. The enhanced swim fin is typically attached to a foot, such as for performance, for training, or for physical therapy. Alternate embodiments of the expandable fin blade may be attached to a hand or to an oar or paddle.